In most conventional ammonia plants, natural gas is processed in primary and secondary reformers to generate hydrogen, and the reformed gas stream is then subjected to a shift conversion for additional hydrogen production after excess heat has been recovered from the reformed gas stream. In a still further step, acid gases (here: carbon dioxide) are removed and residual carbon monoxide (CO) and carbon dioxide (CO2) are converted into methane in a downstream methanator. The resulting raw synthesis gas stream is then passed into the synthesis loop for production of ammonia, wherein the nitrogen is typically provided from process air that is fed into the secondary reformer.
Typically, an ammonia plant will use a stoichiometric amount of process air in the secondary reformer to maintain a hydrogen to nitrogen molar ratio of 3 to 1 in the methanator effluent gas (raw synthesis gas), which is typically the make-up gas to the ammonia synthesis loop. Where desirable, production capacity can be increased by introducing excess air and then by removing excess nitrogen from the syngas stream by combination of gas expansion, autorefrigeration, and cryogenic separation as for example described in U.S. Pat. No. 3,442,613 to Grotz. While Grotz's configuration is relatively effective in some ammonia production plants, retrofitting of existing plants is problematic since the expansion step requires considerable increase in front-end pressure in a ammonia synthesis plant and thereby typically necessitates retrofits/rebuilds to accommodate for the increased pressure.
To circumvent at least some of the problems associated with retrofitting existing ammonia synthesis production plants, Bhakta describes in U.S. Pat. No. 5,935,544 a configuration in which a purifier process has been applied to a low methane content syngas with moderate N2 excess, wherein an external refrigeration step produces a syngas with sufficiently low inert content to significantly increase synthesis capacity or decrease in synthesis loop pressure. Although Bhakta's configuration overcomes in many cases the need for retrofitting existing plants, Bhakta's configuration is typically limited to relatively narrow process parameters in order to provide satisfactory improvements in synthesis capacity.
In yet other known plant configurations (see e.g., our commonly owned International patent application with publication number WO 03/002459), an ammonia plant includes a separation system upstream of a synthesis loop that removes excess nitrogen and other gaseous compounds from a feed gas having a ratio of hydrogen to nitrogen of less than 3:1 to thereby produce a syngas with a ratio of hydrogen to nitrogen at about 3:1. In particularly preferred systems, a coldbox with a refrigerant other than air, or pressure swing adsorption unit operates as the separation system. While such configurations provide numerous advantages over previously known ammonia plants, demands for excess air are still relatively high.
Therefore, despite numerous known configurations and processes for improvements of ammonia synthesis, all or almost all of them suffer from one or more disadvantages. Thus, there is still a need to provide improved methods and configurations for ammonia synthesis.